This invention relates to a method and apparatus for removing processing liquid from a lens-bearing container during manufacture of molded ophthalmic lenses. This invention is suited to molded ophthalmic lenses such as hydrogel contact lenses, although the method is also suitable for other hydrophilic, high-precision ophthalmic lenses such as intraocular lenses. In particular, this method and apparatus are directed to the removal of deionized water from an ophthalmic lens package after lens inspection, and immediately prior to saline dosing and package sealing.
As the ophthalmic lens industry has grown, and in particular the industry related to supplying contact lenses that are provided for periodic frequent replacement, the number of lenses that need to be produced has increased dramatically. This has required manufacturers to strive for methods and apparatus that can be adapted to automated practices and perform with consistency.
Similarly, the promise of easier insertion of a folded or rolled intraocular lenses through a smaller incision has increased the interest in soft intraocular lenses for patients undergoing cataract removal and lens replacement. Soft ophthalmic lenses for placement on the cornea or within the eye, such as contact lenses or soft intraocular lenses, can be made by a variety of techniques. Ophthalmic lenses can be made by spin casting a monomer material in a rotating mold then polymerizing the material so shaped. Another method used to manufacture both contact lenses and intraocular lenses in precision lathing of a piece of material which is then polished and used as a lens.
Recently the molding of soft contact lenses and soft intraocular lenses has come into favor. This technique has the advantages of repeatability and speed that compares favorably with the prior methods of manufacturing lenses, such as by forming a monomer or monomer mixture in a mold such as one mad polystyrene or polypropylene.
Techniques for successfully molding such lenses can be found in U.S. Pat. Nos. 4,495,313 and 4,640,489 to Larsen and U.S. Pat. Nos. 4,889,664; 4,680,336 and 5,039,459 to Larsen et al. These patents specifically described the use of a diluent, a material which substitutes for water during the molding process, and which is replaced with water after the molding has been completed. The advantage of this technique is that the optical properties, size and shape of the lens thus made does not change as radically as with methods that do not utilize such diluent.
It is further known in the art to mold such ophthalmic lenses by forming a monomer or monomer mixture in a mold such as one made form polystyrene or polypropylene.
An example of this art can be found in U.S. Pat. No. 4,565,348 to Larsen. In contrast to the above polystyrene molds, another example is the use of polypropylene or polyethylene molds such as that described in U.S. Pat. No. 4,121,896 to Shepherd.
A practical method and apparatus for mass production of molded contact lenses using the above described processes is given in U.S. Pat. Nos. 5,094,609 and 5,080,839 both to Kindt-Larsen. Although the method and apparatus described therein are specifically directed to the removal of the diluent from the polymerized lens after molding and replacement with water, there is described in general the process steps that must be undertaken subsequent to the removal of the diluent and hydration of the lens.
In the U.S. Pat. No. 5,080,839 patent there is shown in FIG. 1 at element 130 a step described as deionized water removal. This removal is shown as taking place in an inspection carrier or final package. Subsequent steps are shown as saline deposition and sealing of the package.
The method and apparatus described in the above patents represents an improvement in the lens hydration process in that only deionized water is used for hydration, that is, release of the lens from the mold and hydrolysis of the diluent, instead of the performing the solution exchange operation with saline solution in a tank batch process. While this method has the benefits of no salt usage representing a cleaner and less corrosion prone system and accelerating processing times due to the deferment of meth-acrylic acid neutralization, it requires that the lens be transferred from the deionized water to a saline system so the lens can equilibrate to its final properties. Further because the inspection process is done in the primary package, it is necessary to exchange the deionized water with saline solution with the lens in the package and without removing the lens.
In the '839 patent it is stated that the deionized water is removed from the recesses of the inspection carrier and replaced with a saline solution which has a pH and osmolality compatible with the tears of the human eye. It is stated that alternately an aliquot of concentrated brine solution may be added to the deionized water such that the final solution has the same pH and osmolality mentioned above. Saline solution is used so that when the user removes the lens from the package, ready to insert the lens on the cornea of the eye, the pH and osmolality of the lens will be balanced with that of the eye and the lens will not irritate the eye when inserted. If the material from which the lens is made has an ionic characteristic, the salts in the saline solution will neutralize that ionic species. The neutralization can be done in the final package on the shelf outside the remainder of the manufacturing process. It may also be possible therefore to insert a small portion of solid sodium chloride salt into the ionized water to allow this stabilization to occur in the package after sealing.
Two considerations, however, make this approach impractical; first, the overall process requires a high degree of accuracy and repeatability. This is particularly difficult do to the small volume of the package cavity which is approximately 1 ml, and wherein a significant percentage in salinity. The direct addition then of either concentrated saline solution or solid sodium chloride salt would result in a significant variation and in final solution concentration.
The second problem with any attempt to partially remove the deionized water and replace it with concentrated saline is the handling of salt solution. As is well known, sodium chloride is corrosive to many materials and represents processing problems such as salt crystal build-up after evaporation causing faulty seals, which is best avoided.
Great difficulties were encountered in achieving complete water removal by various methods. One such method was the insertion of a needle into the lens container to withdraw water. Experimentation with this method resulted in a repeatability for the six sigma range of 60 mg, which is approximately equal to a variability of 6% dilution of the final packing solution. Another method was the tilting of the package. In both instances, the amount of water on the remaining in the package afterward was variable and could not be relied upon to give the appropriate salinity with the addition of concentrated saline. A second problem with the tilting method is the presence of water of the remainder of the package interfering with final package sealing. Other methods to induce removal of the water in vapor form, such as microwave-induced evaporation, are too slow.
It was an object of the invention described in the parent application to provide a method and apparatus that could quickly and completely remove the deionized water form the lens container. While in large part this apparatus was successful, it was found that in a small percentage of cases, particularly if the lens was not initially well centered, the lens would be dislocated by the jet of air, and slide to the side of the bowl, or onto the nozzle face, and then be dislodged from the package during subsequent material handling. This created, over a period of time, when several lenses had been dislodged, and partially dried by air, a sticky debris which had the ability to create equipment malfunctions, and expensive downtime for the entire line, while the missing, and largely transparent, lenses were located and cleaned from the system. While the error rate was quite small, and less than 1%, the downtime was unduly expensive when the line as a whole was producing several thousand lenses an hour.
It is a further object of the present invention to perform such deionized water removal with a higher success rate, without damaging or altering the final physical properties of the lens.
It is another object of the present invention to provide a method of establishing a final solution concentration that is highly accurate and repeatable so that the lens equilibrates to the proper oslmality.
It is a final object of the invention to provide a method and apparatus that achieves the above objects in such a controlled manner that the liquid is not distributed elsewhere so that moisture interferes with the subsequent sealing of the package.